1. Field of the Invention
The present invention relates to R.sub.2 M.sub.17 (where R represents at least one of rare earth elements including yttrium and M is mainly transition metals) type permanent magnet composition and, more particularly, to the R.sub.2 M.sub.17 type permanent magnet composition whose energy product is improved by increasing its residual magnetic flux density while maintaining its coercive force at a level equal to that obtainable in the prior art.
2. Description of Prior Art
In conventional R.sub.2 M.sub.17 type permanent magnets using samarium as the rare earth element R and cobalt as the transition metal M the copper content is relatively large, above 10 weight percent, to obtain high coercive force (iHc) and iron is added to suppress deterioration of residual magnetic flux density (Br) which is caused when the copper content becomes large. In this instance, the amount of iron added is held less than about 8 weight percent, because the iron, if added in large quantity, would lower the residual magnetic flux density (Br).
However, the energy product (BH) obtainable with such a permanent magnet composition is only 22.1 MG.Oe or so at the largest.
To increase the energy product (BH), a variety of permanent magnet compositions have been proposed so far.
Of the proposed compositions, (1) a composition which contains 22 wt % R, 5 to 12 wt % copper, 0.2 to 5 wt % X (which is at least one of niobium, zirconium, vanadium, tantalum, chromium, hafnium), 0.2 to 8 wt % manganese and the balance being cobalt which is substituted by less than 35 wt % iron (Japanese Patent Publication No. 56-11378), (2) a composition which contains 22 to 28 wt % R, 2 to 10 wt % copper, 6 to 35 wt % T (which is at least one of iron manganese and chromium), 0.5 to 6 wt % M (zirconium and/or hafnium) and the balance being cobalt (Japanese Patent Publication No. 62-61665), and (3) a composition which is represented by the formula R (Co.sub.l-u-v-w Cu.sub.u Fe.sub.v M.sub.w).sub.z where 0&lt;u.ltoreq.2, 0.01&lt;v.ltoreq.0.6, 0.005 .ltoreq.w.ltoreq.0.05, 6.5.ltoreq.z.ltoreq.8.8, and M is at least one element selected from the group consisting of tantalum, zirconium, niobium, titanium and hafnium (Japanese Patent Publication No. 61-17881) are high in both coercive force and residual magnetic flux density, and consequently, provide a great energy product.
All these compositions reduce the copper content but instead call for the addition of such expensive and difficult-to-get elements as tantalum, niobium and hafnium--this inevitably leads to advanced cost of material and hence eventually raises the manufacturing costs of products. Moreover, these compositions are all intended to provide a greater energy product by increasing both of the coercive force and the residual magnetic flux density. However, depending on the elements used, the coercive force increases while decreasing the residual magnetic flux density and vice versa as referred to above. Accordingly, it is very difficult to determine the particular composition which can raise both of characteristics.